Put the raspberries in a stainless steel pan with the water and caster sugar over a low to moderate heat and leave them to simmer for 7 -10 minutes until they are soft. Once there is plenty of juice remove from the heat.

Place the biscuits into the bottom of a large serving bowl, breaking as necessary to fit. Spoon the hot raspberries and their juice over the sponge and leave to cool. During this time the sponge will soak up much of the juice.

Put the egg yolk and sugar in a bowl and mix it well, then stir in the mascarpone and vanilla.

Whip the cream, then, when it is thick enough to lie in soft folds (rather than stand in stiff peaks) fold it lightly into the mascarpone mixture.

In a separate bowl beat the egg white until it is almost stiff, then fold it into the mixture.

Spoon the mascarpone cream over the cool raspberries and sponge fingers. You can smooth it flat or not. Refrigerate for a good hour or so before serving, so that the whole thing has time to come together.